The only thing is, I don't really see why an owner would choose to be a moderator etc.
I can make sense of the fact that main owners may set some GControl settings, like ban time, and then test it as a moderator (without having 2 accounts). However, I owners don't really have access to these settings. You could argue that owners may want to test bot commands and stuff which could be true.
I guess it could be added to GControl but with the addition you mentioned (you can only make yourself a lower rank). That way main owners can choose if they want their staff members to be able to change ranks.